The present invention relates to a sprocket for roller chain that is designed to modulate the engagement frequency of a conventional roller chain without introducing harsh impact loads on the rollers or bushings. The present invention modulates the roller engagement by randomizing the radial seating position of the roller while maintaining a constant chordal length between the seated rollers. In particular, the root diameters of the sprocket roots (or the portion between the sprocket teeth for seating of the rollers) are randomized around the circumference of the sprocket.
Silent chains have previously used randomized sprockets in an attempt to modulate the engagement frequency of the chain links with the sprocket. The randomization of the sprocket is conventionally achieved by relieving, or removing, a small amount of material from the sides of some of the sprocket teeth. The relieved sprocket teeth are typically interspersed with the non-relieved sprocket teeth in a predetermined pattern around the sprocket. The resulting chain drive produced a modulation of noise from engagement at the pitch frequency. An example of such a chain is shown in U.S. Pat. No. 3,377,875.
As a silent chain contacts such a conventional randomized sprocket, the chordal position of the links varies from sprocket tooth to sprocket tooth. As a result, some links in the chain are subjected to higher impact loads when they engage a non-relieved sprocket tooth that follows a succession of relieved teeth.
A roller chain that utilized such a sprocket with relieved teeth would have some rollers subjected to higher impact loads, which would lead to premature fatigue failure of the rollers. Since roller fatigue is one of the prime failure modes of high speed roller chain drives, this problem presents a major obstacle to the adoption of conventional random sprockets for roller chains.
U.S. Pat. No. 3,448,629 is directed to reduction of chain impact in a roller chain by modifying the bottom diameter of the sprocket root from the theoretically correct bottom diameter relative to a given chain and widening the gap between adjacent teeth of the sprocket sufficiently to allow the modified sprockets to mesh with the chain. This sprocket does not provide a randomization of the root diameters of the sprocket roots around the circumference of the sprocket.
PCT International Publication Number WO 97/11292, published Mar. 27, 1997, discloses a roller chain timing drive directed to reduction of impact noise by thinning the teeth and elongating the root land between each pair of teeth. Through detailed calculation, the root land is elongated resulting in a reduced thickness of the teeth as measured across itself from a drive flank to a coast flank. The result is the maintenance of contact of all rollers wrapped around a sprocket throughout the design life of the chain.